In biology and ecology, extinction is the ceasing of existence of a species or group of species. The moment of extinction is generally considered to be the death of the last individual of that species. Extinction is a natural phenomenon; it is estimated that 99.9% of all species that have ever lived are now extinct. In the theory of evolution, new species are created by speciation — where new organisms arise and thrive when they are able to find and exploit an ecologicalniche — and species become extinct when are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance, although some species survive virtually unchanged for hundreds of millions of years.

Descendants may or may not exist for extinct species. Daughter species that evolve from a parent species carry on most of the parent species' genetic information, and even though the parent species may become extinct, the daughter species lives on. In other cases, species have produced no new variants, or none that are able to survive the parent species' extinction.

Extinction of a parent species where daughter species or subspecies are still alive is also called pseudoextinction. Many of prehistoric extinct species have evolved into new species; for example the extinct Eohippus (an ancient horse-like animal) was the ancestor of several extant species including the horse, the zebra and the donkey. The Eohippus itself is no more, but its decendants live on. It is therefore said to be pseudoextinct.

Currently, many environmental groups and governments are concerned with the extinction of species due to human intervention, and are attempting to combat further extinctions. Humans can cause extinction of a species through overharvesting , pollution, destruction of habitat, introduction of new predators, and other influences. According to the World Conservation Union (WCU, formerly the IUCN), 784 extinctions have been recorded since the year 1500 (the arbitrary date selected to define "modern" extinctions), with many more likely to have gone unnoticed. Most of these modern extinctions can be attributed directly or indirectly to human effects. Endangered species are species that are in danger of becoming extinct; several organizations attempt to preserve recognized endangered species through a variety of conservation programs.

Definition

A species becomes extinct when the last existing member of that species dies. Extinction therefore becomes a certainty when no surviving specimens are able to reproduce and create a new generation. A species may become functionally extinct when only a handful of individuals survive, which are unable to reproduce due to health, age, lack of both sexes (in species that reproduce sexually), or other reasons.

In addition to actual extinction, human attempts to preserve critically endangered species have caused the creation of the conservation statusextinct in the wild. Species listed under this status by the WCU are not known to have any living specimens in the wild, and are maintained only in zoos or other artificial environments. Some of these species are functionally extinct. When possible, modern zoological institutions attempt to maintain a viable population for species preservation and possible future reintroduction to the wild through use of carefully planned breeding programs.

Pinpointing the extinction or pseudoextinction of a species requires a clear definition of that species. The species in question must be identified uniquely from any daughter species, as well as its ancestor species or other closely related populations, if it is to be declared extinct. For further discussion, see definition of species.

Permanence

Until recently, it had been universally accepted that the extinction of a species meant the end of its time on Earth. However, recent technological advances have encouraged the hypothesis that through the process of cloning, extinct species may be "brought back to life." Proposed targets for cloning include the mammoth and thylacine. In order for such a program to succeed, a sufficient number of individuals would need to be cloned (in the case of sexually reproducing organisms) to create a viable population size. The cloning of an extinct species has not yet been attempted, due to technological limitations as well as ethical and philosophical questions.

This concept was fictionalized in the popular novel and movie Jurassic Park.

Causes

There are a variety of causes that can contribute directly or indirectly to the extinction of a species or group of species. Most simply, any species that is unable to survive or reproduce in its environment, and unable to move to a new environment where it can do so, dies out and becomes extinct. Extinction of a species may come suddenly when an otherwise healthy species is wiped out completely, as when toxic pollution renders its entire habitat unlivable; or may occur gradually over thousands or millions of years, such as when a species gradually loses out competition for food to newer, better adapted competitors.

Genetic and demographic causes

Genetic and demographic phenomena affect the evolution, and therefore extinction, of species. Regarding the possibility of extinction, small populations which represent an entire species are much more vulnerable to these types of effects.

Natural selection acts to propagate beneficial genetic traits and eliminate weaknesses. However, it is sometimes possible for a deleterious mutation to be spread throughout a population through the effect of genetic drift.

A diverse or "deep" gene pool gives a population a higher chance of surviving an adverse change in conditions. Effects that cause or reward a loss in genetic diversity can increase the chances of extinction of a species. Population bottlenecks can dramatically reduce genetic diversity by severly limiting the number of reproducing individuals and make inbreeding more frequent. The founder effect can cause rapid, individual-based speciation and is the most dramatic example of a population bottleneck.

Habitat degradation

The degradation of a species' habitat may alter the fitness landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur by direct effects, such as the environment becoming toxic, or indirectly, by limiting a species' ability to compete effectively for diminished resources or against new competitor species.

Habitat degradation through toxicity can kill off a species very rapidly, by killing all living members through contamination or sterilizing them. It can also occur over longer periods at lower toxicity levels by affecting life span, reproductive capacity, or competitiveness. DDT played such a role in killing off bald eagles and other birds by thinning the egg shell walls of affected birds, thus lowering the survivability of offspring. Since this effect was discovered, DDT has been banned in many parts of the world and affected bird populations are recovering.

Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely sited as an example of this; elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fern that depends on dense shade for protection from direct sunlight can no longer survive with no forest to house it.

Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming has allowed some species to expand their range, bringing unwelcome competition to other species that previously occupied that area. Sometimes these new competitors are predators and directly affect prey species, while at other times they may merely outcompete vulnerable species for limited resources.

Vital resources including water and food can also be limited during habitat degradation, causing some species to become extinct.

Predation, competition, and disease

Coextinction

Effects

Mass extinctions

There have been at least five mass extinctions in the history of life, in which many species have disappeared in a relatively short period of geological time. These are covered in more detail in the article on extinction events. The most recent of these, the K-T extinction 65 million years ago at the end of the Cretaceous period, is best known for having wiped out the non-feathereddinosaurs, called the non-avian dinosaurs, among many other species.

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70 percent of biologists believe that we are currently in the early stages of a human-caused mass extinction, known as the Holocene extinction event. In that survey, the same proportion of respondents agreed with the prediction that up to 20 percent of all living species could become extinct within 30 years (by 2028). Biologist E.O. Wilson, Professor Emeritus and honorary curator of entomology at Harvard University, in his 2002 book The Future of Life (ISBN 0679768114), estimates that if current rates of human destruction of the biosphere continue, one-half of all species of life on earth will be extinct in 100 years.

See also: The Sixth Extinction : Patterns of Life and the Future of Humankind by Richard Leakey ( ISBN 0385468091 ).

Human extinction

Human extinction refers to the possibility that the human species may become extinct. Although humans are relatively intelligent and adaptive, they could cause an extinction through their own actions, or allow a natural extinction event to occur through negligence.

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